Figuring out what happens to sequestered carbon

Carbon sequestration has been proposed as a method of limiting the …

The energy economy remains reliant on fossil fuels, which is leading to the continued accumulation of CO2 in the atmosphere. But it may be possible to continue burning fossil fuels without affecting the atmosphere; pilot studies are already underway to evaluate the potential of techniques that extract CO2 from the exhaust stream of power plants. Of course, once the carbon is removed, there's still the issue of what to do with it afterwards. Most plans involve placing it in geological formations that are already known to trap gasses: those that we've extracted natural gas from, to be precise.

Chemically, however, CO2 has very different properties from hydrocarbons, so it's not guaranteed to behave in the same way once we stick it underground. Even a small rate of release, less than one percent annually, would mean that sequestration would simply delay any problems associated with high levels of atmospheric carbon dioxide. So it's essential that we have some idea of what might happen to underground reservoirs of CO2. A paper that appears in today's issue of Nature takes a big step in that direction by exploring the fate of some naturally occurring CO2 reservoirs.

Various geological processes release CO2, and it sometimes gets trapped in the same deposits as natural gas. The authors of the new paper have obtained information about the gas composition of five fields in the Rocky Mountains, along with two in China and one each in Texas and Hungary. By measuring the isotope ratios in samples taken from these locations, they were able to make some inferences as to what's happening to the CO2.

The first key step was an examination of the ratio of CO2 to the 3He, an isotope of helium, in these deposits. The helium is chemically inert, and should remain relatively constant. The authors found a wide range of ratios between the two gasses, suggesting that something was happening with the more reactive carbon dioxide.

The next key observation was that the decline in the relative value of CO2 was proportional to an increase in the amount of a different inert gas, neon. The most likely source of the neon is water that has picked up small amounts of the gas during contact with the atmosphere, and then exchanged it with the gas in the deposits. This, in turn, suggests that the reduction in CO2 was related to the presence of water. Similar results held for a different isotope of helium, 4He.

There are two ways for water to remove CO2 from the gaseous state. The first is good from the perspective of sequestration: once dissolved in water, the gas can undergo reactions in which it precipitates out in the form of carbonates. The other option is simply that it remains dissolved as the water circulates elsewhere so that there is never an equilibration between the CO2 in the water and in the gaseous state. The researchers attempted to distinguish between the two by examining the amount of a specific isotope of carbon, 13C. This slightly heavier isotope is more likely to precipitate out, so the fraction of carbon dioxide that contains it should shrink if precipitation is occurring.

The results suggest that, in many cases, the ratio between the two carbon isotopes is constant, so no carbonates are forming in the water. In a few exceptions, where the gas deposits are associated with specific classes of rocks, some precipitation is happening, but it accounts for less than 20 percent of the CO2 reduction.

The net conclusion is that, over time, sequestered carbon is likely to wind up dissolved in the subsurface waters near where we pump it. That's not necessarily a bad thing—if that water doesn't exchange it with the atmosphere, or does so at a slow rate, sequestration would remain a viable option. What it does tell us is that we need to start studying the dynamics of the underground water system near any geological features that are being considered for this use.

Carbon sequestration can be very dangerous (deadly even). Lake Nyos was very rich in CO2 a seismic event caused a rapid release of the CO2 which formed a cloud that spread to villages all around which killed almost everyone and everything in the vicinity. http://www.geology.sdsu.edu/ho...anoes_work/Nyos.html I don't want to be anywhere near a place that's storing this stuff.